Have the tires filled up ~75% with a calcium chloride / water solution. Fill to the top of the rim, which allows for expansion, but do not exceed this level. Calcium chloride drops the freeze point level to subzero temperatures (-20 or so dependant upon solution ratio), allows for additional weighting, (about 10-11 lbs. per gal. / a few hundred additional lbs. per tire), and does not adversely affect handling except in one minor situation. Do not put anything other than air in the front wheels of 4WD tractors, or tractor hop (an ugly shuddering) can occur at field speeds. Filling 2WD front tires will not result in this problem. This should be available from any reputable tractor supplier. This infomation has been well studied by U.C.Davis and other ag research groups and has been determined to be a good alternative to using mounted weights.